The lithography using radiation of very short wavelength such as soft x-ray makes fine image writing possible at the sacrifice of focal depth. For printing a fine precise pattern, the substrate for the reflective mask is required to meet the completeness of surface parameters including high flatness, low defect density, and minimal surface roughness.
It is contemplated to use a synthetic quartz glass substrate having a high flatness as the mask substrate for permitting the current lithography using ArF excimer laser (wavelength 193 nm) to survive until the transition to the extreme ultraviolet lithography (EUVL).
At the present, substrates are finished to a high flatness necessary to accommodate fine patterns of the EUVL or the like. Specifically, they are required to have a flatness of up to 50 nm within a central region of 142 mm squares on the substrate surface. Several versions of substrates have been proposed as meeting the requirement. For example, JP-A 2007-287737 discloses a highly flat and highly smooth substrate obtained by local processing such as by gas cluster ion beam etching and finish polishing. JP-A 2004-291209 discloses a method for producing a highly flat and low defective substrate through local machining by plasma etching and subsequent non-contact polishing. However, the above methods for flattening substrates are inconvenient in the apparatus size and process flow, and the eventual increases in processing cost and time are outstanding problems. For example, the gas cluster ion beam etching requires a time-consuming step of creating a vacuum environment prior to the start of processing, and the non-contact polishing such as float polishing is disadvantageous in that a longer polishing/processing time is needed due to a low polishing rate. A greater capital of investment needed for the large size of apparatus and operation expenses including expensive gases necessary for the processing are reflected by the cost of substrate processing so that substrates may become more expensive. Increases in the price of glass substrates are unfavorable to both the supply and demand sides.
WO 2004083961 describes a substrate in which the maximum height from a reference plane at the boundary between a main surface and a chamfered surface and the shape are specified so that the substrate may be improved in flatness when vacuum chucked. However, the substrate has a flatness of 0.2 at best as described in the patent document. By taking into account a shape change when the substrate is mounted on the stepper by suction chucking, a substrate is designed to have a peripheral portion which is flat or outward inclined. However, with this area setting, flatness control is difficult because a shape change of the peripheral portion has a less influence on flatness within the effective range. Accordingly, the proposal is short of capability for high-flatness substrates such as substrates for the EUVL that imposes a severe requirement on the flatness of substrate surface.
Even if a substrate is prepared to a high flatness sufficient to accommodate not only the ArF excimer laser lithography, but also the EUVL, the flatness is broken upon mounting of the substrate on the stepper by suction chucking. There is a need for a glass substrate having a shape which has taken into account a shape change of the substrate by suction chucking. In order to comply with the EUV lithography, the substrate must meet a flatness of up to 50 nm over the effective range when the substrate is mounted on the stepper by suction chucking.